1. Field of the Invention
The present invention relates to a zoom lens for suitable use in an image taking optical system of an image pickup apparatus such as a video camera, a digital still camera, a broadcast camera, or a silver film camera.
2. Description of the Related Art
Recently, image pickup apparatuses (cameras) using solid-state image pickup elements, such as video cameras and digital still cameras, have become smaller with increased functionality. As an image taking optical system used in a camera, a zoom lens that has a high zooming ratio and that can be used in the camera while allowing a compact size of the camera is demanded.
In order both to reduce the size of a camera and to increase the zooming ratio of a zoom lens, a so-called retractable zoom lens is used. In the retractable zoom lens, while image taking is not performed, the distances between lens units are reduced so as to be different from those during an image taking operation.
In another type of zoom lens, a reflective member (prism) for folding an image taking light beam 90° in the optical path is provided in the optical path in order to reduce the thickness of the camera (in the front-rear direction of the camera set for image taking) (U.S. Pat. Nos. 7,206,139 and 6,754,446 and Japanese Patent Laid-Open No. 2004-102089).
According to U.S. Pat. Nos. 7,206,139 and 6,754,446 and Japanese Patent Laid-Open No. 2004-102089, light from an object is folded 90° relative to the optical axis of a lens unit by a prism having a reflective surface, so that the thickness of the camera is reduced so as to be substantially equal to the length between an object-side lens unit and the prism.
A four- or five-unit zoom lens is known as a zoom lens that is used with a high zooming ratio in an image pickup apparatus (Japanese Patent Laid-Open No. 61-123811 and U.S. Pat. No. 6,987,622).
For example, in a known zooming lens, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a negative refractive power, and a rear lens unit including at least one lens unit and having a positive refractive power as a whole are arranged from an object side to an image side.
When a zoom lens in which a reflective member for folding light from the object side is provided in the optical path is applied to a camera, the size of the camera in the thickness direction (camera thickness) can be reduced easily.
However, when the zoom lens including the reflective member provided in the optical path is applied to the camera, it is important to properly set the lens configuration of the zoom lens in order to reduce the camera thickness and to increase the zooming ratio of the zoom lens. In particular, it is important to place the reflective member at a proper position in the optical path.
If the lens configuration of the zoom lens and the position of the reflective member in the optical path are improper, the size (effective diameter) of the reflective member increases, the strokes (moving amounts) of the lens units during zooming for a high zooming ratio increase, and the size of the entire lens system increases. As a result, when the zoom lens is applied to the camera, it is difficult to reduce the camera thickness.
U.S. Pat. No. 7,206,139 discloses a four-unit zoom lens having a zooming ratio of about 6. In this zoom lens, lens units respectively having positive, negative, positive, and positive refractive powers are arranged in that order from the object side to the image side, and a reflective member for folding the optical path is provided in a second lens unit.
Unfortunately, in the zoom lens disclosed in U.S. Pat. No. 7,206,139, when the zooming ratio is further increased, the stroke of a first lens unit relevant to the camera thickness increases, and this makes it difficult to reduce the camera thickness.
In a four-unit zoom lens disclosed in U.S. Pat. No. 6,754,446, lens units respectively having positive, negative, positive, and positive refractive powers are arranged, and a reflective member for folding the optical path is provided in a first lens unit closest to the object side. For this reason, there is a tendency for the reflective member to be large.
In many cases, the size of the image-pickup apparatus in the thickness direction is determined by the size of the reflective member. Therefore, it is not preferable that the reflective member be large.
Japanese Patent Laid-Open No. 2004-102089 discloses a four- or five-unit zoom lens in which lens units having positive, negative, positive, and positive refractive powers or lens units having positive, negative, positive, positive, and positive refractive powers are arranged in that order from the object side to the image side. A reflective member for folding the optical path is provided in a second lens unit.
In the four- or five-unit zoom lens, displacement of the image plane resulting from movement of an image-side lens unit for zooming is corrected by a lens unit closest to the image side, and a zooming ratio of about 10 is achieved by greatly moving a first lens unit to the object side.
Accordingly, the stroke of the first lens unit during zooming is long, and this makes it difficult to reduce the thickness of the camera to which the zoom lens is applied.
In zoom lenses disclosed in Japanese Patent Laid-Open No. 61-123811 and U.S. Pat. No. 6,987,622, lens units having positive, negative, negative, and positive refractive powers or lens units having positive, negative, negative, positive, and positive refractive powers are arranged in that order from the object side to the image side. However, a reflective member for folding the optical path is not used in these zoom lenses.
Therefore, it is difficult to reduce the thickness of the camera when the zoom lens is applied thereto.